Laser brazing method and production method for lap joint member

ABSTRACT

The present invention provides a method for laser brazing an Al alloy to a steel sheet so as to produce a lap joint member that excels in bonding strength with respect to the Al alloy. The laser brazing method includes a brazing step of forming a brazed part ( 9 ) for bonding an Al alloy ( 3 ) to a hot dip Zn-based alloy coated steel sheet ( 1 ) in which a coating layer ( 2 ) contains 1.0 to 22.0% by mass of Al and a coating weight per surface is 15 to 250 g/m 2 .

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a U.S. national phase application, pursuant to 35U.S.C. § 371, of PCT/JP2018/006219, filed Feb. 21, 2018, which claimspriority to Japanese Application No. 2017-031611, filed Feb. 22, 2017and Japanese Application No. 2017-193611, filed Oct. 3, 2017. The entirecontents of the aforementioned patent applications are incorporatedherein by this reference.

TECHNICAL FIELD

The present invention relates to (i) a method for laser brazing Al or anAl alloy to a hot dip Zn-based alloy coated steel sheet, and (ii) amethod for producing a lap joint member with the laser brazing method.

BACKGROUND ART

In order to reduce a weight of an automobile body, an amount of used Aland Al alloy (hereinafter, Al and an Al alloy are collectively referredto as “Al alloy”) is increasing. However, in a case where an Al alloy isbonded to a steel sheet with a conventional melt welding process such asan arc welding process, there is a problem that a fragile Fe—Al-basedintermetallic compound phase grows thicker in an interface between thesteel sheet and a weld bead, and therefore bonding strength decreases.

As a method for inhibiting growth of the Fe—Al-based intermetalliccompound phase, for example, Patent Literatures 1 through 4 disclosesteel sheets for brazing in which concentrations of C, Si, and Mn insteel of an alloyed hot dip Zn coated steel sheet, a hot dip Zn coatedsteel sheet, and a Zn electroplated steel sheet are specified. The steelsheet for brazing diffuses, from the steel, C, Si, and Mn into theFe—Al-based intermetallic compound phase in brazing, and thus inhibitsgrowth of the fragile Fe—Al-based intermetallic compound phase, andheightens the bonding strength.

Si and Mn are useful in heightening strength of steel sheets, and areused in high-tensile steel sheets. However, in a hot dip Zn coated steelsheet, Si and Mn are incrassated into oxides on a steel sheet surface inreduction heating before plating. This inhibits wettability between asteel sheet and a plating bath, and causes a plating defect. In analloyed hot dip Zn coated steel sheet, Si and Mn inhibit diffusion thatoccurs between a steel sheet and a coating layer in an alloying processafter plating, and thus causes a delay in alloying reaction.

In brazing also, Si and Mn in steel are incrassated on the steel sheetsurface in brazing, and inhibit wettability between a brazing fillermetal and an Al alloy. As a result, repelling between the brazing fillermetal and the Al alloy, pits, and blowholes are generated, and thiscauses deterioration in bead appearance, detachment of bead, anddecrease in bonding strength. Therefore, amounts of Si and Mn to beadded to steel are limited. Moreover, strength of the steel sheetincreases due to the addition of Si and Mn in steel as above, andtherefore a type of steel to which such steel is applicable is limitedto high-tensile steel, and such steel cannot be used in a general steelsheet.

CITATION LIST Patent Literature

[Patent Literature 1]

Japanese Patent Application Publication Tokukai No. 2006-283110(Publication date: Oct. 19, 2006)

[Patent Literature 2]

Japanese Patent Application Publication Tokukai No. 2006-283111(Publication date: Oct. 19, 2006)

[Patent Literature 3]

Japanese Patent Application Publication Tokukai No. 2007-247024(Publication date: Sep. 27, 2007)

[Patent Literature 4]

Japanese Patent Application Publication Tokukai No. 2007-277717(Publication date: Oct. 25, 2007)

[Patent Literature 5]

Japanese Patent Application Publication Tokukai No. 2008-68290(Publication date: Mar. 27, 2008)

[Patent Literature 6]

Japanese Patent Application Publication Tokukai No. 2011-218424(Publication date: Nov. 4, 2011)

SUMMARY OF INVENTION Technical Problem

An object of the present invention is to provide a technique to producea lap joint member that excels in bead appearance and in bondingstrength by using, in laser brazing an Al alloy to a steel sheet, a hotdip Zn-based alloy coated steel sheet in which Al is added to a coatinglayer.

Solution to Problem

As a result of diligent study by the inventors of the present invention,the inventors have newly found that a hot dip Zn-based alloy coatedsteel sheet in which a coating layer contains Al excels in affinity witha brazing filler metal and an Al alloy and, in a case where the Al alloyis brazed, it is possible to achieve good bead appearance and highbonding strength. Based on this novel finding, the inventors haveaccomplished the present invention.

That is, a laser brazing method in accordance with an aspect of thepresent invention is a method for laser brazing a bonding target sheetthat is stacked on a sheet surface of a hot dip Zn-based alloy coatedsteel sheet, a coating layer of the hot dip Zn-based alloy coated steelsheet containing 1.0% by mass to 22.0% by mass of Al, and the bondingtarget sheet being an Al sheet or an Al alloy sheet, the methodincluding: a brazing step of forming a brazed part on the sheet surfacefor joining the bonding target sheet and the hot dip Zn-based alloycoated steel sheet together.

Advantageous Effects of Invention

According to an aspect of the present invention, it is possible toprovide the laser brazing method for producing a lap joint member whichis provided between the Al alloy and the steel sheet and excels in beadappearance and in bonding strength.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic view illustrating a cross section of a lap jointmember in an embodiment of the present invention.

FIG. 2 is a schematic view illustrating an example of a position atwhich a brazing filler metal abuts on an Al alloy in a laser brazingmethod in accordance with Embodiment 1 of the present invention.

FIG. 3 is a schematic view illustrating a state in which the abuttingposition is at an upper limit of a range defined in the presentinvention.

FIG. 4(a) is a plan view illustrating a position at which the brazingfiller metal abuts on the Al alloy in the laser brazing method inaccordance with Embodiment 1 of the present invention, and FIG. 4(b) isa cross-sectional view taken along the line A-A in FIG. 4(a).

FIG. 5 is a plan view illustrating a laser brazing method in accordancewith Embodiment 3 of the present invention.

DETAILED DESCRIPTION

The following description will discuss embodiments of the presentinvention. Note that the following descriptions are aimed merely atbetter understanding of the gist of the invention, and do not limit thepresent invention unless otherwise specified. In the presentapplication, “A to B” means A or more (higher) and B or less (lower).

Embodiment 1

Before now, various measures have been proposed for inhibiting growth ofan Fe—Al-based intermetallic compound phase in brazing. However, thereare cases where the growth inhibiting effect is insufficient, and a typeof steel of a coated steel sheet to which such a measure is applicableis limited. Under the circumstances, the inventors of the presentinvention carried out the diligent study and, as a result, the followingnew conception has been obtained. That is, a hot dip Zn-based alloycoated steel sheet in which a Zn-based coating layer contains Al excelsin affinity with a brazing filler metal and an Al alloy. This broadens abead width W (later described), and it is therefore possible to increasea shear plane area of a bead. From this, the inventors of the presentinvention have found that it is possible to prevent a decrease inbonding strength (shear strength) of the brazed part even in a casewhere the Fe—Al-based intermetallic compound phase has grown thicker.Based on this new finding, the inventors of the present invention haveaccomplished the present invention. The following description willdiscuss details of an embodiment of the present invention.

[Hot Dip Zn-Based Alloy Coated Steel Sheet]

According to a hot dip Zn-based alloy coated steel sheet for brazing inaccordance with an embodiment of the present invention, a coating layercontains Zn as a main component, and contains Al in an amount of notless than 1.0% by mass and not more than 22.0% by mass. In general, inbrazing of an Al alloy, a brazing filler metal is used which contains Alor Zn as a main component. According to the hot dip Zn-based alloycoated steel sheet for brazing in accordance with an embodiment of thepresent invention, the coating layer contains (i) an Al alloy and (ii)Zn and Al which are main components of a brazing filler metal.Therefore, the hot dip Zn-based alloy coated steel sheet in accordancewith Embodiment 1 excels in affinity with the Al alloy and the brazingfiller metal. Further, wettability is improved and it is thereforepossible to achieve good bead appearance. Furthermore, the bead width isbroadened, and this enhances the bonding strength.

This effect is obtained when the Al concentration is not less than 1% bymass, and therefore the Al concentration is limited to not less than 1%by mass.

In contrast, in a case where the Al concentration exceeds 22% by mass, amelting point of the Zn-based coating layer becomes higher and theaffinity is deteriorated. In this case, the wettability is deteriorated,and a humping bead with irregular bead widths occurs, that is, the beadappearance is deteriorated. Moreover, the bead width is narrowed, andthe bonding strength is deteriorated.

In a case where 0.05% by mass to 10.0% by mass of Mg is added to theZn-based coating layer, a melting point of the Zn-based coating layerbecomes lower. Further, the affinity with the Al alloy and the brazingfiller metal improves, and this leads to excellent bonding strength.Moreover, the addition of Mg also leads to excellent corrosionresistance. Therefore, it is preferable to add 0.05% by mass to 10.0% bymass of Mg to the Zn-based coating layer. In a case where Mg is added,the coating layer preferably contains Ti: 0.002% by mass to 0.1% by massor B: 0.001% by mass to 0.05% by mass in order to inhibit generation andgrowth of a Zn₁₁Mg₂ system phase which causes deterioration in coatinglayer appearance and in corrosion resistance.

Further, the Zn-based coating layer can contain Si in an amount of notmore than 2.0% by mass in order to inhibit growth of the Fe—Al-basedintermetallic compound phase in brazing and accordingly to heighten thebonding strength. Moreover, the Zn-based coating layer can contain Fe inan amount of not more than 2.5% by mass.

The above feature can be expressed as follows. That is, the coatinglayer of the hot dip Zn-based alloy coated steel sheet for brazing inaccordance with an embodiment of the present invention preferablycontains Zn as a main component, contains 1% by mass to 22% by mass ofAl and 0.05% by mass to 10.0% by mass of Mg, and further satisfies oneor more conditions selected from the group consisting of Ti: 0.002% bymass to 0.1% by mass, B: 0.001% by mass to 0.05% by mass, Si: 0% by massto 2.0% by mass, and Fe: 0% by mass to 2.5% by mass.

[Coating Weight]

According to the hot dip Zn-based alloy coated steel sheet for brazingin accordance with an embodiment of the present invention, it is moreeffective to set a coating weight per surface to 15 g/m² or more inorder to achieve good wettability with respect to the Al alloy and thebrazing filler metal. The coating weight per surface can also beexpressed as a coating weight on the sheet surface of the hot dipZn-based alloy coated steel sheet on which sheet surface the brazed partis formed. A low coating weight is disadvantageous for maintainingcorrosion resistance and sacrificial anti-corrosive effect of a coatedsurface over a long period of time. Therefore, the coating weight is setto 15 g/m² or more.

In contrast, in a case where the coating weight per surface exceeds 250g/m², a generation amount of Zn vapor in brazing becomes greater, andthis leads to generation of a humping bead, pits, and blowholes, andaccordingly leads to deterioration in bead appearance and in bondingstrength. Therefore, the coating weight is preferably 250 g/m² or less.

[Al Alloy]

According to an embodiment of the present invention, an Al alloy is notlimited in particular. It is possible to use a 5000-series Al alloy or a6000-series Al alloy which is mainly used in an automobile body, and theother Al alloy can also be used. For example, it is possible to use a1000-series Al alloy, a 3000-series Al alloy, or a 7000-series Al alloy.Note that, as described earlier, the term “Al alloy” used in thisspecification encompasses pure Al (that may contain impurities).

In this specification, unless otherwise noted, the Al alloy has a shapeof sheet, and accordingly the Al alloy means an Al alloy sheet. Further,according to an embodiment of the present invention, the Al sheet or theAl alloy sheet is to be subjected to laser brazing, and therefore can beexpressed as a bonding target sheet.

[Bead Width]

FIG. 1 is a schematic view illustrating a cross section of a lap jointmember in which the hot dip Zn-based alloy coated steel sheet forbrazing in accordance with an embodiment of the present invention isbrazed to an Al alloy. A lap joint is a joint that is often used inbrazing an Al alloy to a steel sheet for an automobile body.

As illustrated in FIG. 1, in the lap joint member in accordance with anembodiment of the present invention, an Al alloy 3 is stacked on a hotdip Zn-based alloy coated steel sheet 1, and a bead 4 is formed bybrazing. The bead 4 can be expressed as a brazed part (brazed jointpart) that is formed by brazing.

A sheet thickness of the Al alloy 3 that is used in the automobile bodyis selected as appropriate in accordance with demanded strength.Therefore, in the present invention, a bead width W is also controlledwithin a range of Formula (2) below in accordance with the sheetthickness of the Al alloy 3. That is, in a case where the sheetthickness is increased so as to enhance strength of the Al alloy 3, thebead width W is also broadened to enhance bonding strength.t≤W≤4t  (2)

where

W: bead width (mm) of brazed part

t: sheet thickness (mm) of Al alloy.

For example, in a case where the sheet thickness of the Al alloy 3 is 2mm, bonding strength adequate for the automobile body is 4 kN or more.In a case where the bead width W satisfies Formula (2) above, it ispossible to achieve excellent bonding strength even if an Fe—Al-basedintermetallic compound phase 5 grows. However, in a case where the beadwidth W is smaller than a sheet thickness t of the Al alloy 3, thebonding strength decreases. In contrast, in a case where the bead widthW exceeds 4 t, it is necessary to increase a heat input for broadeningthe bead width, and consequently an evaporation region of a Zn-basedcoating layer 2 around a bead part becomes larger, and thereforecorrosion resistance decreases.

[Brazing Method]

The lap joint member in accordance with an embodiment of the presentinvention is brazed with use of a laser brazing method. The followingdescription will discuss the laser brazing method in accordance with anembodiment of the present invention, with reference to FIG. 2 and FIG.3.

FIG. 2 is a view schematically illustrating the laser brazing method inaccordance with an embodiment of the present invention. As illustratedin FIG. 2, the Al alloy 3 is stacked on the hot dip Zn-based alloycoated steel sheet 1, and a brazing filler metal 7 abuts on an endsurface 6 of the Al alloy 3. For example, the brazing filler metal 7 canbe in a form of long rod (i.e., cylinder) or can be a wire.Alternatively, for example, the brazing filler metal 7 can have aplate-like shape. According to an aspect of the present invention, thebrazing filler metal 7 and the hot dip Zn-based alloy coated steel sheet1 are arranged such that a distance d (mm) between the brazing fillermetal 7 and a sheet surface of the hot dip Zn-based alloy coated steelsheet 1 satisfies Formula (1) below.0.2≤d≤t  (1)

where

t: sheet thickness (mm) of Al alloy.

More specifically, the distance d is a shortest distance between thesheet surface of the hot dip Zn-based alloy coated steel sheet 1 and asurface (i.e., an outer edge of a cross section of the brazing fillermetal 7 that is taken along a plane orthogonal to a longitudinaldirection of the brazing filler metal 7) of the brazing filler metal 7.Note that the sheet surface of the hot dip Zn-based alloy coated steelsheet 1 can be expressed as a surface of the Zn-based coating layer 2.The brazing filler metal 7 is arranged such that the longitudinaldirection of the brazing filler metal 7 goes along the end surface 6 ofthe Al alloy 3 and Formula (1) above is satisfied.

A method for arranging the brazing filler metal 7 so as to satisfyFormula (1) above is not particularly limited. For example, it ispossible to use a holding member (not illustrated) for holding thebrazing filler metal 7. Note, however, that it is preferable to avoidusing a fixing agent (e.g., adhesive agent) or the like for temporarilyfixing an abutting part between the brazing filler metal 7 and the endsurface 6 of the Al alloy 3. This is for preventing deterioration incharacteristic of a bead that is to be formed by brazing. In otherwords, the brazing filler metal 7 is preferably held so as to physicallyabut on the end surface 6 of the Al alloy 3 (i.e., such that pressingforce is generated in a direction from the brazing filler metal 7 to theend surface 6).

In a case where the brazing filler metal 7 is apart from the end surface6 of the Al alloy 3, only the brazing filler metal 7 is melted, and theAl alloy 3 is not melted. Therefore, the brazing filler metal 7 iscaused to abut on the end surface 6 of the Al alloy 3.

Zn which is a main component of the Zn-based coating layer 2 of the hotdip Zn-based alloy coated steel sheet 1 has a boiling point of 907° C.,which is near to a melting point 660° C. of Al. Therefore, in meltingthe Al alloy 3 and the Al-based brazing filler metal 7, Zn may beevaporated because a temperature of the Zn-based coating layer 2 becomeshigher than the boiling point of Zn. In a case where a large amount ofthe Zn vapor is generated and ejected from a molten bead, a humping beador pits may be caused, and this leads to deterioration in beadappearance. Moreover, in a case where the Zn vapor is confined in abead, the Zn vapor becomes blowholes and a bonding area is reduced, andthis leads to a decrease in bonding strength.

In view of this, according to an aspect of the present invention, thedistance d (mm) between the brazing filler metal 7 and the sheet surfaceof the hot dip Zn-based alloy coated steel sheet 1 is set to 0.2 mm ormore as indicated in Formula (1) above for discharging the Zn vapor. Anupper limit of the distance d corresponds to a sheet thickness t of theAl alloy 3, that is, the upper limit is represented by d=t. In thepresent invention, “d=t” means a state in which the brazing filler metal7 abuts on a corner portion of the Al alloy 3 as illustrated in FIG. 3.In a case where the distance d is equal to or greater than the sheetthickness of the Al alloy 3, the brazing filler metal 7 is to apart fromthe Al alloy 3 and therefore, as above described, only the brazingfiller metal 7 is melted and the Al alloy 3 is not melted.

As the brazing method, a laser brazing method is suitably employed. Alaser beam has high energy density. Therefore, in a case where the laserbeam is used in brazing, a brazing rate becomes higher, and further anevaporation region of the Zn-based coating layer in the vicinity of abead is narrowed, and this makes it possible to maintain high corrosionresistance. Further, laser brazing is carried out by selective heating,and this allows reduction in heat distortion.

The following description will discuss more details of the laser brazingmethod in accordance with Embodiment 1, with reference to FIG. 4(a) andFIG. 4(b).

FIG. 4(a) is a plan view illustrating a position at which the brazingfiller metal abuts on the Al alloy in Embodiment 1. FIG. 4(b) is across-sectional view taken along the line A-A in FIG. 4(a).

As illustrated in FIG. 4(a) and FIG. 4(b), the Al alloy 3 is stacked onthe hot dip Zn-based alloy coated steel sheet 1, and the brazing fillermetal 7 is arranged to abut on the end surface 6 of the Al alloy 3. Inthat state, a laser beam 8 is emitted toward the brazing filler metal 7.A bead 4 (see FIG. 1) for joining the hot dip Zn-based alloy coatedsteel sheet 1 and the Al alloy 3 together is formed by moving anirradiation position with the laser beam 8 in a direction in which thebrazing filler metal 7 extends.

[Brazing Filler Metal]

In Embodiment 1, a material of the brazing filler metal 7 is notparticularly limited. It is possible to employ a typical Al-basedbrazing filler metal or Zn-based brazing filler metal for Al alloy.Moreover, in order to inhibit growth of the Fe—Al-based intermetalliccompound phase, Si can be added to the brazing filler metal.Alternatively, it is possible to use a flux brazing filler metal whichcontains flux for improving wettability.

[Target Location of Laser]

As a target location of laser, the laser beam 8 emitted is targeted onthe brazing filler metal 7. The brazing filler metal 7 abuts on the endsurface of the Al alloy while satisfying the relation indicated byFormula (1) above, and the Al alloy 3 is stacked on the hot dip Zn-basedalloy coated steel sheet 1. From this, in a case where the laser beam isemitted toward the brazing filler metal 7 so as to melt the brazingfiller metal 7, not only the brazing filler metal 7 but also the endsurface 6 of the Al alloy 3 and the coating layer 2 of the hot dipZn-based alloy coated steel sheet 1 are melted. Thus, the Al alloy 3 isbrazed to the hot dip Zn-based alloy coated steel sheet 1. Further, thebrazing filler metal 7 satisfies the relation of Formula (1), andtherefore the Zn vapor is surely discharged. Therefore, a humping bead,pits, and blowholes are less likely to be generated by brazing.

[Beam Diameter]

The laser beam 8 preferably has a beam diameter that is wider than thatin typical laser welding. For example, the beam diameter of the laserbeam 8 is preferably greater than a diameter of the brazing filler metal7. That is, the beam diameter of the laser beam 8 is preferably set tofall within a range of D1 or more and 4 t or less, where “D1” is thediameter of the brazing filler metal 7 and “t” is the sheet thickness ofthe Al alloy 3. In a case where the laser beam 8 is targeted on andemitted to the brazing filler metal 7, the brazing filler metal 7 isentirely heated up. From this, the brazing filler metal 7 is melted, andalso the end surface 6 of the Al alloy 3 and the coating layer 2 of thehot dip Zn-based alloy coated steel sheet 1 are melted. Thus the Alalloy 3 is brazed to the hot dip Zn-based alloy coated steel sheet 1.

Moreover, the laser beam 8 more preferably has a beam diameter withwhich the brazing filler metal 7 and the end surface 6 of the Al alloy 3are both irradiated with the laser beam 8. That is, the beam diameter ofthe laser beam 8 is more preferably (i) greater than a length obtainedby adding up the diameter of the brazing filler metal 7 and the sheetthickness of the Al alloy 3 (i.e., a width of the end surface 6) and(ii) set to fall within a range of (D1+t) or more and 4 t or less. Fromthis, it is possible to heat up both the brazing filler metal 7 and theend surface 6 of the Al alloy 3, and it is also possible to entirelyheat up a part in which the brazed part 2 is formed by laser brazing. Asa result, Zn vapor is more likely to be discharged from between thebrazing filler metal 7 and the sheet surface of the hot dip Zn-basedalloy coated steel sheet 1. Moreover, a thermal gradient in a heataffected zone is more likely to be gentle, and this makes it possible toreduce generation of heat distortion.

Advantageous Effect

As such, according to the laser brazing method in accordance with anaspect of the present invention, it is possible to produce the lap jointmember that excels in bead appearance and in bonding strength.

In general, a steel sheet may be bonded to an Al alloy by spot welding.As compared with such spot welding, the laser brazing method inaccordance with Embodiment 1 has the following advantages. That is,unlike the welding in which the steel sheet is melted and bonded, it issufficient to melt the coating layer 2 and the brazing filler metal 7 inthe present laser brazing method, and it is not necessary to melt a basesteel sheet of the hot dip Zn-based alloy coated steel sheet 1. Fromthis, a quantity of heat used for bonding is smaller. Consequently, itis possible to reduce heat distortion that would occur in the lap jointmember. Therefore, for example, it is possible to reduce a problem suchas warpage that may occur in the spot welding.

Further, by the present laser brazing method, it is possible tosuccessively bond the hot dip Zn-based alloy coated steel sheet to theAl alloy. Therefore, the lap joint member that has been produced by thepresent laser brazing method excels in sealing property. For example, inthe field of automobiles, a gap is filled with a sealer after spotwelding. In a case where the present laser brazing method is employed,it is unnecessary to carry out such a process of using a sealer.

Further, according to the present laser brazing method, it is possibleto make the Fe—Al-based intermetallic compound phase thinner which isformed in the joint part, and it is also possible to improve bondingstrength of the lap joint member by the foregoing successive bonding.

Moreover, according to the present laser brazing method, it is possibleto reduce generation of blowholes and spatter, as compared with a casewhere brazing is carried out with use of an arc.

Example 1-1

The present invention will be described below in detail with referenceto Examples and Comparative Examples. Note, however, that the presentinvention is not limited to such Examples.

A hot dip Zn-based alloy coated steel sheet having a sheet thickness of1.2 mm, a width of 200 mm, and a length of 100 mm, and a 6000-series Alalloy member having a sheet thickness of 2.0 mm, a width of 200 mm, anda length of 100 mm were prepared. Then, laser brazing was carried outwith respect to the Al alloy member that was stacked on the hot dipZn-based alloy coated steel sheet with a positional displacement of 50mm in a length direction, and thus a lap joint sample was prepared.Table 1 shows laser brazing conditions.

TABLE 1 Laser welding machine Disk laser welding machine Laser output 1to 4 kW Beam diameter 5 mm Brazing filler metal Al-1.0% by mass of Si,Fluoride-based flux wire, 1.2 mmϕ Distance (d) between brazing filler 0to 2.5 mm metal and hot dip Zn-based alloy coated steel sheet Shieldinggas Ar gas, flow rate: 20 L/min Brazing rate 0.5 to 2 m/min

The lap joint sample thus prepared was visually observed, and thepresence or absence of humping and pits was checked. After the visualobservation, X-ray radiography was carried out to check the presence orabsence of blowholes. After the X-ray radiography, a sample for tensiletest having a width of 30 mm was taken out from a central part of thelap joint sample, and a tensile test was carried out at a tensile speedof 3 mm/min. In the present invention, bonding strength is defined by amaximum load in the tensile test. Bonding strength adequate for theautomobile body is 4 kN or more. A sample for observing a cross sectionwas taken out from a part very near to the part from which the samplefor tensile test was taken out, and cross section observation wascarried out with use of a microscope to check a bead width W.

The investigation results of the laser brazing method are shown in Table2. In regard to the bead appearance in Table 2, a successive beadappearance without humping and pits was evaluated as good, and a case inwhich humping or pits were generated was evaluated as bad.

In Examples in which laser brazing was carried out within the range ofthe present invention indicated by No. 1 through No. 13 in Table 2,humping and pits were not generated, and good bead appearances wereobtained. Moreover, the bonding strength was 4 kN or more, and thus thebonding strength sufficient for an automobile body was obtained.

In Comparative Example No. 14 in which an Al concentration of thecoating layer was smaller than the range of the present invention, thebead width W was smaller, i.e., 0.5 mm, and the bonding strength wasinsufficient, i.e., 1.3 kN.

In Comparative Example No. 15 in which the coating weight exceeded therange of the present invention and in Comparative Example No. 16 inwhich the distance d between the hot dip Zn-based alloy coated steelsheet and the brazing filler metal was 0 mm (i.e., a state in which thebrazing filler metal is making contact with the hot dip Zn-based alloycoated steel sheet), humping and pits were generated due to influence ofZn vapor, and therefore the bead appearance was notably deteriorated,and blowholes were generated. In Comparative Examples No. 15 and No. 16,the bead appearance was notably deteriorated, and therefore the tensiletest was not carried out.

In Comparative Example No. 17 in which the distance d between the hotdip Zn-based alloy coated steel sheet and the brazing filler metalexceeded the range of the present invention, only the brazing fillermetal was melted, and the Al alloy was not melted. Therefore, the X-rayradiography and the tensile test were not carried out.

In Comparative Examples No. 18 through No. 20, the bead width W wassmaller than the range of the present invention, and the bondingstrength was insufficient, i.e., 3.4 kN or less.

TABLE 2 Distance d (mm) Coating between brazing weight filler metal andhot Tensile Coating layer composition (% by mass) per surface dipZn-based alloy Bead Bead width test result No. Al Mg Ti B Si Fe (g/m²)coated steel sheet appearanc Blowhole W (mm) (kN) Class 1 1.1 — — — — —15 0.3 Good None 3.2 4.6 Example 2 4.0 — — — — — 45 0.5 Good None 5.15.3 3 21.7  — — — — 90 0.6 Good None 6.0 5.5 4 1.2  0.07 — — — — 120 1.2 Good None 5.8 5.4 5 2.1 2.0 — — — 150  2.0 Good None 6.7 6.9 6 4.11.0 — — — — 180  0.5 Good None 7.8 7.6 7 21.8  9.7 — — — 220  0.2 GoodNone 2.2 4.0 8 6.1 3.0 — — — — 245  0.4 Good None 4.5 4.4 9 5.9 3.10.035 — — — 90 1.6 Good None 6.6 6.2 10 6.0 3.0 0.034 0.02  0.056  0.0290 2.0 Good None 7.3 7.1 11 11.1  3.2 — — — — 90 1.6 Good None 6.1 5.512 11.0  3.0 — — 0.054  0.01 90 1.8 Good None 8.0 7.8 13 21.8  5.9 0.1 0.05  2.0  2.5 90 1.3 Good None 4.3 5.0 14 0.2 — — — — — 15 0.3 GoodNone 0.5 1.3 Com. 15 1.1 — — — — — 275  0.3 Bad Exist — — Example 1622.0  — — — — 160  0   Bad Exist — — 17 5.8 3.3 0.002 0.005 — — 90 2.5Bad — — — 18 6.0 2.8 0.002 0.001 — — 45 1.3 Good None 1.6 3.4 19 5.1 3.00.025 0.01  0.025  0.002 90 1.5 Good None 1.2 2.6 20 5.8 3.0 0.1  0.05 1.8  2.5 120  1.2 Good None 1.4 3.1

Example 1-2

Laser brazing was carried out with use of a hot dip Zn-based alloycoated steel sheet and each of Al alloys of 1000-series, 3000-series,5000-series, 6000-series, and 7000-series. Table 3 shows compositions ofthose Al alloys.

TABLE 3 Composition (% by mass) Type of Al alloy Si Fe Cu Mn Mg Cr Zn TiAl 1000-series 0.01 0.02 0.01 0.01 0.01 — 0.01 0.01 Bal. 3000-series0.53 0.21 0.13 0.88 0.054 0.01 0.01 0.01 Bal. 5000-series 0.09 0.10 0.020.07 4.63 0.09 0.16 0.02 Bal. 6000-series 0.66 0.33 0.26 0.08 0.96 0.180.01 0.01 Bal. 7000-series 0.22 0.13 1.56 0.12 2.66 0.21 5.54 0.02 Bal.

Specifically, a hot dip Zn-based alloy coated steel sheet having a sheetthickness of 1.2 mm, a width of 200 mm, and a length of 100 mm, and anAl alloy member of each series having a sheet thickness of 1.0 mm to 3.0mm, a width of 200 mm, and a length of 100 mm were prepared. Then, laserbrazing was carried out with respect to the Al alloy member that wasstacked on the hot dip Zn-based alloy coated steel sheet with apositional displacement of 50 mm in a length direction, and thus a lapjoint sample was prepared. Laser brazing conditions are similar to thosein Table 1 above.

Table 4 shows the results.

TABLE 4 Distance d (mm) between brazing filler Coating metal and weightSheet hot dip Bead Coating layer per thickness Zn-based width Tensilecomposition (% by mass) surface Type of (mm) of alloy coated Bead W testresult No. Al Mg Ti B Si Fe (g/m²) Al alloy Al alloy steel sheetappearance Blowhole (mm) (kN) Class 21 6.0 3.0 0.034 0.02 0.056 0.02 901000- 1.0 0.3 Good None 1.2 2.8 Example 22 series 2.0 1.9 Good None 6.16.4 23 3.0 2.9 Good None 11.7 10.1 24 3000- 1.0 0.9 Good None 2.0 3.9 25series 2.0 1.1 Good None 7.6 7.4 26 3.0 0.2 Good None 10.9 8.6 27 5000-1.0 0.5 Good None 2.3 4.1 28 series 2.0 0.3 Good None 6.3 5.8 29 3.0 1.6Good None 9.9 7.8 30 6000- 1.0 0.9 Good None 1.8 3.4 31 series 2.0 2.0Good None 7.3 7.1 32 3.0 3.0 Good None 11.2 9.0 33 7000- 1.0 0.4 GoodNone 2.2 4.1 34 series 2.0 0.5 Good None 7.3 7.5 35 3.0 2.1 Good None9.5 8.2

In Examples in which laser brazing was carried out within the range ofthe present invention indicated by No. 21 through No. 35 in Table 4,humping and pits were not generated, and good bead appearances wereobtained. Moreover, the bonding strength corresponded to the type andthe sheet thickness of each of the Al alloys, and thus the bondingstrength sufficient for an automobile body was obtained.

Embodiment 2

The following description will discuss another embodiment of the presentinvention. For convenience of explanation, identical reference numeralsare given to constituent members having functions identical with thoseof the constituent members described in the foregoing embodiment, anddescriptions of such constituent members are not repeated here.

Conventionally, as a method for bonding an Al alloy to a steel sheetwhile inhibiting growth of an Fe—Al-based intermetallic compound phase,a flux wire brazing process is employed in which an Al alloy wirecontaining flux that contains fluoride, chloride, or the like as a maincomponent is used. The flux containing fluoride, chloride, or the likeis used in order to activate the steel sheet surface by removing oxideon the steel sheet surface with an etching effect, and to achieve goodadherence by improving wettability between (i) the steel sheet and (ii)a melted Al alloy and a melted Al alloy wire.

For example, Patent Literature 5 discloses a method for brazing an Alalloy to a hot dip Zn coated steel sheet with use of an Al alloy wirethat contains Si in an amount of 1% by mass to 13% by mass and alsocontains flux containing AlF₃. According to the brazing method, goodbonding strength is supposed to be obtained by (i) an activation effectby the flux on the steel sheet surface and (ii) an effect of inhibitinggrowth of an Fe—Al-based intermetallic compound phase by Si in the Alalloy wire and AlF₃ in the flux.

However, in a case where the flux wire is used, a flux residue remainsin a bead part after brazing, and this deteriorates an appearance.Furthermore, the flux residue serves as a corrosion accelerating factor,and therefore corrosion resistance is notably deteriorated. Furthermore,in a case where painting is carried out after brazing, paintability andcorrosion resistance after painting are also notably deteriorated.Therefore, in a case where the flux wire is used, not only a cost ofwire increases but also a cost of process increases because the fluxresidue needs to be cleaned off.

A fluxless brazing method in which no flux is used can be, for example,a method disclosed in Patent Literature 6. Patent Literature 6 disclosesan arc brazing method in which a location to be brazed is irradiatedwith a laser beam in advance to evaporate oxide, and sequentiallybrazing is carried out while using a brazing filler metal as aconsumable electrode. According to the method, a laser heating deviceand an arc-welding device are required, and therefore a cost ofequipment is high, and accordingly a cost of brazing increases.

Before now, various methods for brazing an Al alloy to a steel sheethave been proposed. However, such conventional methods have a problemthat flux needs to be added to a wire and a flux residue needs to beremoved, and also have a problem of increase in cost, particularlyincrease in cost of equipment due to using of a laser heating device incombination with an arc-welding device.

An object of Embodiment 2 is to provide a lap joint member which excelsin bead appearance and in bonding strength at a low cost by carrying outfluxless laser brazing for brazing an Al alloy to a steel sheet with useof a hot dip Zn-based alloy coated steel sheet in which a coating layercontains Al.

The inventors of the present invention carried out the diligent studyand, as a result, the following new conception has been obtained. Thatis, the inventors have newly found that a hot dip Zn-based alloy coatedsteel sheet in which a Zn-based coating layer contains Al excels inwettability between a brazing filler metal and an Al alloy, andtherefore it is possible to obtain a lap joint that excels in bondingstrength by carrying out laser brazing without using flux. Based on thisnew finding, the inventors of the present invention have accomplishedthe present invention. The following description will discuss details ofan embodiment of the present invention.

[Hot Dip Zn-Based Alloy Coated Steel Sheet]

According to a hot dip Zn-based alloy coated steel sheet that is used ina method for producing a lap joint member of Embodiment 2 by carryingout fluxless laser brazing on the hot dip Zn-based alloy coated steelsheet and an Al alloy member, a coating layer contains Zn as a maincomponent, and contains Al in an amount of not less than 1.0% by massand not more than 22.0% by mass. The coating layer of the hot dipZn-based alloy coated steel sheet contains Al which is a main componentof the Al alloy and of the brazing filler metal, and this allows thecoating layer of the hot dip Zn-based alloy coated steel sheet to excelin wettability between the Al alloy and the brazing filler metal. It istherefore possible to achieve good bead appearance without using flux,and the bead width is broadened, and this enhances the bonding strength.This effect is obtained when the Al concentration is not less than 1% bymass, and therefore the Al concentration is limited to not less than 1%by mass. In contrast, in a case where the Al concentration exceeds 22%by mass, a melting point of the Zn-based coating layer becomes higherand wettability is deteriorated. Further, a humping bead with irregularbead widths occurs and accordingly the bead appearance is deteriorated.Moreover, the bead width is narrowed, and the bonding strength isdeteriorated.

In a case where 0.05% by mass to 10.0% by mass of Mg is added to the hotdip Zn-based alloy coating layer, a melting point of the Zn-basedcoating layer becomes lower. Further, the wettability between the Alalloy and the brazing filler metal improves without using flux, and thisleads to excellent bonding strength. Moreover, the addition of Mg alsoleads to excellent corrosion resistance. Therefore, it is preferable toadd 0.05% by mass to 10.0% by mass of Mg to the hot dip Zn-based alloycoating layer. In a case where Mg is added, the coating layer cancontain Ti: 0.002% by mass to 0.1% by mass or B: 0.001% by mass to 0.05%by ma ss in order to inhibit generation and growth of a Zn₁₁Mg₂ systemphase which causes deterioration in coating layer appearance and incorrosion resistance.

Further, the Zn-based coating layer can contain Si in an amount of notmore than 2.0% by mass in order to inhibit growth of the fragileFe—Al-based intermetallic compound phase in brazing and accordingly toheighten the bonding strength. Moreover, the Zn-based coating layer cancontain Fe in an amount of not more than 2.5% by mass.

The above feature can be expressed as follows. That is, the coatinglayer of the hot dip Zn-based alloy coated steel sheet that is used inthe method for producing a lap joint member by fluxless laser brazing inaccordance with an aspect of the present invention contains Zn as a maincomponent and contains Al in an amount of 1% by mass to 22% by mass.Preferably, the coating layer satisfies one or more conditions selectedfrom the group consisting of: Mg: 0.05% by mass to 10.0% by mass, Ti:0.002% by mass to 0.1% by mass, B: 0.001% by mass to 0.05% by mass, Si:0% by mass to 2.0% by mass, and Fe: 0% by mass to 2.5% by mass.

[Al Alloy]

The Al alloy that is used in fluxless laser brazing of Embodiment 2 issimilar to that used in Embodiment 1, and therefore the description ofthe Al alloy is omitted here.

[Brazing Method]

The fluxless laser brazing method in Embodiment 2 is similar to that ofEmbodiment 1 but is different in the following point. That is, inEmbodiment 2, a fluxless brazing filler metal is used as a brazingfiller metal 7 (see FIGS. 2 and 3). This means that the brazing fillermetal used in the fluxless laser brazing in Embodiment 2 contains noflux.

The following description will discuss the fluxless brazing fillermetal.

[Brazing Filler Metal]

The brazing filler metal in accordance with Embodiment 2 can be composedof Al. Alternatively, the brazing filler metal can be an alloy having acomposition containing one or more selected from Si: 0.2% by mass to15.0% by mass and Mn: 0.03% by mass to 2.0% by mass, in addition to Al.An effect brought about by adding Si or Mn is to inhibit growth of anFe—Al-based intermetallic compound phase in a bead part. Further, thebrazing filler metal in accordance with an aspect of the presentinvention can contain Mg in an amount of 0.3% by mass to 7.0% by mass inorder to lower a melting point and viscosity of the brazing filler metaland to improve wettability between (i) the brazing filler metal and (ii)the Al alloy and the hot dip Zn-based alloy coated steel sheet.

A shape of the brazing filler metal in Embodiment 2 can be a wire or,for example, a plate-like shape. The shape of the brazing filler metalis preferably a shape with which the distance d between the brazingfiller metal and the hot dip Zn-based alloy coated steel sheet can bemanaged so as to satisfy the relation of Formula (1) below in a state inwhich the brazing filler metal abuts on the end surface of the Al alloy.0.2≤d≤t  (1)

where

t: sheet thickness (mm) of Al alloy

[Coating Weight, Bead Width]

A coating weight per surface of the hot dip Zn-based alloy coated steelsheet and a bead width of the lap joint member in accordance withEmbodiment 2 are similar to those of Embodiment 1, and therefore thedescriptions are omitted here.

Advantageous Effect

According to the fluxless laser brazing method in accordance withEmbodiment 2, the following effect is brought about, in addition to aneffect similar to that of the laser brazing method described inEmbodiment 1.

That is, the coating layer of the hot dip Zn-based alloy coated steelsheet contains Al which is a main component of the Al alloy and of thebrazing filler metal, and this allows the coating layer to excel inwettability between the Al alloy and the brazing filler metal. It istherefore possible to achieve good bead appearance without using flux,and the bead width can be broadened, and this can enhance the bondingstrength.

As such, it is unnecessary to add flux to the brazing filler metal, andit is also unnecessary to carry out a process of removing (cleaning) aflux residue after brazing. Further, as compared with the conventionalmethod in which a laser heating device is used in combination with anarc-welding device, a cost of equipment is lower. Therefore, it ispossible to reduce a cost for brazing.

Example 2-1

The following shows Examples of Embodiment 2.

A hot dip Zn-based alloy coated steel sheet having a sheet thickness of1.2 mm, a width of 200 mm, and a length of 100 mm, and a 6000-series Alalloy member having a sheet thickness of 2.0 mm, a width of 200 mm, anda length of 100 mm were prepared. Then, fluxless laser brazing wascarried out with respect to the Al alloy member that was stacked on thehot dip Zn-based alloy coated steel sheet with a positional displacementof 50 mm in a length direction, and thus a lap joint sample wasprepared. Table 5 shows laser brazing conditions. Table 6 shows detailsof the brazing filler metal.

TABLE 5 Laser welding machine Disk laser welding machine Laser output 1to 4 kW Beam diameter 5 mm Diameter of brazing filler metal 1.2 mmϕDistance (d) between brazing filler 0 to 2.5 mm metal and hot dipZn-based alloy coated steel sheet Shielding gas Ar gas, flow rate: 20L/min Brazing rate 0.5 to 2 m/min

TABLE 6 Type Composition Pure Al Purity: 99.0% by mass Al—Si Si: 0.2 to15.0% by mass Al—Mn Mn: 0.03 to 2.0% by mass Al—Mg Mg: 0.3 to 7.0% bymass Al—Si—Mn—Mg Si: 0.2 to 15.0% by mass Mn: 0.03 to 2.0% by mass Mg:0.3 to 7.0% by mass

The lap joint sample thus prepared was visually observed, and thepresence or absence of humping and pits was checked. After the visualobservation, X-ray radiography was carried out to check the presence orabsence of blowholes. After the X-ray radiography, a sample for tensiletest having a width of 30 mm was taken out from a central part of thelap joint sample, and a tensile test was carried out at a tensile speedof 3 mm/min. In the present invention, bonding strength is defined by amaximum load in the tensile test. Bonding strength adequate for theautomobile body is 4 kN or more. In the lap joint sample, a sample forobserving a cross section was taken out from a part adjacent to the partfrom which the sample for tensile test was taken out, and cross sectionobservation was carried out with use of a microscope to check a beadwidth W.

Table 7 shows results of investigating the lap joint sample that wasprepared by the fluxless laser brazing. In regard to the bead appearancein Table 7, a successive bead appearance without humping and pits wasevaluated as good, and a case in which humping or pits were generatedwas evaluated as bad.

TABLE 7 Coating weight per Brazing filler metal Coating layercomposition (% by mass) surface composition (% by mass) No. Al Mg Ti BSi Fe (g/m²) Si Mn Mg Al 36 1.2 — — — — — 15 — — — Pure Al 37 4.1 — — —— — 45 0.2 — — Bal. 38 21.5 — — — — 60 14.8  — — Bal. 39 1.2  0.08 — — —— 120 —  0.03 — Bal. 40 2.1 1.9 — — — 150 — 2.0 — Bal. 41 4.2 1.0 — — —— 180 — — 0.4 Bal. 42 21.8 9.3 — — — 210 — — 6.9 Bal. 43 6.0 3.0 — — — —245 2.1 1.0 — Bal. 44 5.9 3.0 0.035 — — — 90 5.8 — 7.0 Bal. 45 6.1 2.90.034 0.019 0.056 0.02 90 4.7 0.9 3.9 Bal. 46 11.1 3.4 — — — — 90 0.2 0.03 0.3 Bal. 47 11.0 3.0 — — 0.054 0.01 90 11.0  1.2 3.5 Bal. 48 21.85.8 0.09  0.05  1.9  2.5  90 15.0  2.0 7.0 Bal. 49 0.1 — — — — — 15 — —— Pure Al 50 1.3 — — — — — 280 7.6 — — Bal. 51 21.9 — — — — 120 — 1.1 —Bal. 52 6.0 3.3 0.002 0.005 — — 90 — — 7.0 Bal. 53 6.3 3.0 0.002 0.002 —— 60 0.3 0.5 0.4 Bal. 54 6.0 2.8 0.025 0.01  0.025 0.002 90 10.5  1.12.6 Bal. 55 5.9 3.0 0.1  0.05  1.8  2.4  120 14.0  1.9 6.8 Bal. Distanced (mm) between brazing filler metal and hot dip Zn-based Bead alloycoated Bead width W Tensile test No. steel sheet appearance Blowhole(mm) result (kN) Class 36 0.2 Good None 2.1 4.0 Example 37 0.4 Good None2.2 4.4 38 0.6 Good None 3.4 5.5 39 1.1 Good None 4.4 4.5 40 1.9 GoodNone 4.9 5.9 41 2.0 Good None 4.8 5.7 42 0.2 Good None 4.3 5.4 43 0.5Good None 4.5 4.4 44 1.5 Good None 5.6 6.2 45 2.0 Good None 6.3 7.3 460.6 Good None 5.1 5.5 47 0.8 Good None 6.0 7.8 48 1.3 Good None 8.0 7.949 0.2 Good None 0.7 1.2 Com. 50 0.3 Bad Exist — — Example 51 0   BadExist — — 52 2.7 Bad — — — 53 1.2 Good None 1.5 3.2 54 1.5 Good None 1.12.0 55 1.2 Good None 1.2 3.0

In Examples in which fluxless laser brazing was carried out within therange of the present invention indicated by No. 36 through No. 48 inTable 7, humping and pits were not generated, and good bead appearanceswere obtained. Moreover, the bonding strength was 4 kN or more, and thusthe bonding strength sufficient for an automobile body was obtained.

In Comparative Example No. 49 in which an Al concentration of thecoating layer was smaller than the range of the present invention, thebead width W was smaller, i.e., 0.7 mm, and the bonding strength wasinsufficient, i.e., 1.2 kN.

In Comparative Example No. 50 in which the coating weight exceeded therange of the present invention and in Comparative Example No. 51 inwhich the distance d between the hot dip Zn-based alloy coated steelsheet and the brazing filler metal was 0 mm (i.e., a state in which thebrazing filler metal is making contact with the hot dip Zn-based alloycoated steel sheet), humping and pits were generated due to influence ofZn vapor, and therefore the bead appearance was notably deteriorated,and blowholes were generated. In Comparative Examples No. 50 and No. 51,the bead appearance was notably deteriorated, and therefore the tensiletest was not carried out.

In Comparative Example No. 52 in which the distance d between the hotdip Zn-based alloy coated steel sheet and the brazing filler metalexceeded the range of the present invention, only the brazing fillermetal was melted, and the Al alloy was not melted. Therefore, the X-rayradiography and the tensile test were not carried out.

In Comparative Examples No. 53 through No. 55, the bead width W wassmaller than the range of the present invention, and the bondingstrength was insufficient, i.e., 3.2 kN or less.

Example 2-2

Fluxless laser brazing was carried out with use of a hot dip Zn-basedalloy coated steel sheet and each of Al alloys of 1000-series,3000-series, 5000-series, 6000-series, and 7000-series (see Table 3).

Specifically, a hot dip Zn-based alloy coated steel sheet having a sheetthickness of 1.2 mm, a width of 200 mm, and a length of 100 mm, and anAl alloy member of each series having a sheet thickness of 1.0 mm to 3.0mm, a width of 200 mm, and a length of 100 mm were prepared. Then,fluxless laser brazing was carried out with respect to the Al alloymember that was stacked on the hot dip Zn-based alloy coated steel sheetwith a positional displacement of 50 mm in a length direction, and thusa lap joint sample was prepared. Fluxless laser brazing conditions weresimilar to those in Table 5 above.

Table 8 shows the results. Moreover, as the fluxless brazing fillermaterial, brazing filler metals having compositions shown in Table 8were used.

TABLE 8 Coating Brazing filler Sheet Coating layer weight per metalcomposition thickness composition (% by mass) surface (% by mass) Typeof (mm) of Al No. Al Mg Ti B Si Fe (g/m²) Si Mn Mg Al Al alloy alloy 565.1 2.9 0.034 0.019 0.056 0.02 90 4.7 0.9 3.9 Bal. 1000- 1.0 57 series2.0 58 3.0 59 3000- 1.0 60 series 2.0 61 3.0 62 5000- 1.0 63 series 2.064 3.0 65 6000- 1.0 66 series 2.0 67 3.0 68 7000- 1.0 69 series 2.0 703.0 Distance d (mm) between brazing Tensile filler metal and hot Beadtest dip Zn-based alloy Bead width result No. coated steel sheetappearance Blowhole W (mm) (kN) Class 56 0.3 Good None 1.6 3.6 Example57 1.0 Good None 4.0 4.0 58 3.0 Good None 9.2 8.8 59 1.0 Good None 2.54.0 60 1.5 Good None 7.8 7.9 61 0.3 Good None 11.6 9.3 62 0.5 Good None2.2 4.4 63 1.9 Good None 6.5 7.4 64 1.6 Good None 10.7 9.9 65 0.2 GoodNone 1.5 3.2 66 2.0 Good None 8.3 7.3 67 3.0 Good None 9.3 8.1 68 0.6Good None 2.4 4.2 69 1.4 Good None 4.0 5.4 70 1.5 Good None 11.5 8.9

In Examples in which fluxless laser brazing was carried out within therange of the present invention indicated by No. 56 through No. 70 inTable 8, humping and pits were not generated, and good bead appearanceswere obtained. Moreover, the bonding strength corresponded to the typeand the sheet thickness of each of the Al alloys, and thus the bondingstrength sufficient for an automobile body was obtained.

Embodiment 3

The following description will discuss another embodiment of the presentinvention with reference to FIG. 5. Note that configurations which arenot described in Embodiment 3 are similar to those described inEmbodiment 1 and Embodiment 2. For convenience of explanation, identicalreference numerals are given to constituent members having functionsidentical with those of the constituent members described in theforegoing embodiments, and descriptions of such constituent members arenot repeated here.

In Embodiments 1 and 2 above, the brazing filler metal 7 is arrangedalong the end surface 6 of the Al alloy 3, and the brazing filler metal7 in that state is irradiated with the laser beam 8. Meanwhile,Embodiment 3 is different from Embodiments 1 and 2 in that an end of thebrazing filler metal abuts on the end surface 6 of the Al alloy 3 andbrazing is carried out by irradiation with the laser beam 8 whilesequentially supplying the brazing filler metal 7.

FIG. 5 is a plan view illustrating a laser brazing method in accordancewith Embodiment 3. As illustrated in FIG. 5, in Embodiment 3, an end ofa brazing filler metal 17 abuts on the end surface 6 of the Al alloy 3,and the end part of the brazing filler metal 17 is irradiated with alaser beam 8. A material of the brazing filler metal 17 can be similarto that of the brazing filler metal 7 described in Embodiments 1 and 2above. Moreover, a beam diameter of the laser beam 8 can be similar tothat described in Embodiment 1, and is preferably greater than a beamdiameter in typical laser welding.

In the laser brazing method of Embodiment 3 also, the distance d (mm)satisfies Formula (1) below.0.2≤d≤t  (1)

where

t: sheet thickness (mm) of Al alloy.

Specifically, the distance d is a shortest distance between an outeredge of the end of the brazing filler metal 17 and the hot dip Zn-basedalloy coated steel sheet 1. The brazing filler metal 17 is consumed as abrazed part 9 is formed to braze the Al alloy 3 to the hot dip Zn-basedalloy coated steel sheet 1, and therefore the brazing filler metal 17 issequentially supplied toward the Al alloy 3.

Moreover, a position (hereinafter, sometimes referred to as “abuttingposition”) at which the brazing filler metal 17 abuts on the end surface6 of the Al alloy 3 and the irradiation position with the laser beam 8move while the brazed part 9 is formed as brazing proceeds. Inaccordance with a brazing rate, a moving speed of the abutting positionand a supplying speed of the brazing filler metal 17 are adjusted. Thebrazed part 9 can also be expressed as bead 4 (see FIG. 1).

From this, it is possible to form the brazed part 9 while supplying thebrazing filler metal 17 without arranging and holding the brazing fillermetal 17 in advance. This improves a degree of freedom of workspace, andthus preparation before brazing becomes relatively easier. As a result,it is possible to shorten a working time. Further, it is possible toadjust an amount of the brazing filler metal 17 that is used to form thebrazed part 9. That is, a volume of the brazed part 9 can be increasedby lowering the moving speed of the abutting position or by increasingthe supplying speed of the brazing filler metal 17. From this, it ispossible to enhance bonding strength between the hot dip Zn-based alloycoated steel sheet 1 and the Al alloy 3.

Aspects of the present invention can also be expressed as follows:

As above described, the laser brazing method in accordance with anaspect of the present invention is a method for laser brazing a bondingtarget sheet that is stacked on a sheet surface of a hot dip Zn-basedalloy coated steel sheet, a coating layer of the hot dip Zn-based alloycoated steel sheet containing 1.0% by mass to 22.0% by mass of Al, andthe bonding target sheet being an Al sheet or an Al alloy sheet, themethod including: a brazing step of forming a brazed part on the sheetsurface for joining the bonding target sheet and the hot dip Zn-basedalloy coated steel sheet together.

According to the laser brazing method in accordance with an aspect ofthe present invention, in the brazing step, the brazed part ispreferably formed while arranging the brazing filler metal such that thebrazing filler metal abuts on an end surface of the bonding target sheetand a distance d (mm) between the brazing filler metal and the sheetsurface satisfies Formula (1) below:0.2≤d≤t  (1)

where

t: sheet thickness (mm) of the bonding target sheet.

According to the laser brazing method in accordance with an aspect ofthe present invention, in the brazing step, the brazed part ispreferably formed such that, in a cross section of the brazed part takenalong a plane orthogonal to a direction in which the brazed partextends, a bead width of the brazed part on the sheet surface satisfiesFormula (2) below:t≤W≤4t  (2)

where

W: bead width (mm) of the brazed part

t: sheet thickness (mm) of the bonding target sheet.

According to the laser brazing method in accordance with an aspect ofthe present invention, in the brazing step, fluxless laser brazing canbe carried out with use of a fluxless brazing filler metal as a brazingfiller metal.

According to the laser brazing method in accordance with an aspect ofthe present invention, it is possible that the brazing filler metal iscomposed of Al or has a composition that contains Al and one or moreselected from the group consisting of Si: 0.2% by mass to 15.0% by mass,Mg: 0.3% by mass to 7.0% by mass, and Mn: 0.03% by mass to 2.0% by mass.

According to the laser brazing method in accordance with an aspect ofthe present invention, the coating layer of the hot dip Zn-based alloycoated steel sheet can further contain Mg in an amount of 0.05% by massto 10.0% by mass.

According to the laser brazing method in accordance with an aspect ofthe present invention, the coating layer of the hot dip Zn-based alloycoated steel sheet preferably further contains one or two or moreselected from the group consisting of Ti: 0.002% by mass to 0.1% bymass, B: 0.001% by mass to 0.05% by mass, Si: 0% by mass to 2.0% bymass, and Fe: 0% by mass to 2.5% by mass.

According to the laser brazing method in accordance with an aspect ofthe present invention, a coating weight on the sheet surface of the hotdip Zn-based alloy coated steel sheet is preferably 15 g/m² to 250 g/m².

The method for producing a lap joint member in accordance with an aspectof the present invention is a method for producing the lap joint memberby brazing a bonding target sheet to a hot dip Zn-based alloy coatedsteel sheet with the foregoing laser brazing method.

The lap joint member in accordance with an aspect of the presentinvention is a lap joint member, in which: a laser brazed part is formedby stacking and laser brazing a bonding target sheet onto a sheetsurface of a hot dip Zn-based alloy coated steel sheet; a coating layerof the hot dip Zn-based alloy coated steel sheet contains 1.0% by massto 22.0% by mass of Al; the bonding target sheet is an Al sheet or an Alalloy sheet; and the laser brazed part is formed such that, in a crosssection of the laser brazed part taken along a plane orthogonal to adirection in which the laser brazed part extends, a bead width of thelaser brazed part on the sheet surface satisfies Formula (2) below:t≤W≤4t  (2)

where

W: bead width (mm) of the laser brazed part

t: sheet thickness (mm) of the bonding target sheet.

According to the lap joint member in accordance with an aspect of thepresent invention, the coating layer of the hot dip Zn-based alloycoated steel sheet preferably further contains Mg in an amount of 0.05%by mass to 10.0% by mass.

The present invention is not limited to the embodiments, but can bealtered by a skilled person in the art within the scope of the claims.The present invention also encompasses, in its technical scope, anyembodiment derived by combining technical means disclosed in differingembodiments. Further, it is possible to form a new technical feature bycombining the technical means disclosed in the respective embodiments.

REFERENCE SIGNS LIST

-   -   1: Hot dip Zn-based alloy coated steel sheet    -   2: Zn-based coating layer (coating layer)    -   3: Al alloy    -   4: Bead (brazed part)    -   5: Fe—Al-based intermetallic compound phase    -   6: End surface of Al alloy    -   7, 17: Brazing filler metal    -   8: Laser beam    -   9: Brazed part

The invention claimed is:
 1. A method for laser brazing a bonding targetsheet that is stacked on a sheet surface of a hot dip Zn-based alloycoated steel sheet, a coating layer of the hot dip Zn-based alloy coatedsteel sheet containing 1.0% by mass to 22.0% by mass of Al, and thebonding target sheet being an Al sheet or an Al alloy sheet, the methodcomprising: a brazing step of forming a brazed part on the sheet surfacefor joining the bonding target sheet and the hot dip Zn-based alloycoated steel sheet together, wherein, in the brazing step, the brazedpart is formed while arranging the brazing filler metal such that thebrazing filler metal abuts on an end surface of the bonding target sheetand a distance d (mm) between the brazing filler metal and the sheetsurface satisfies Formula (1) below:0.2≤d≤t  (1) where t: sheet thickness (mm) of the bonding target sheet.2. The method as set forth in claim 1, wherein: in the brazing step, thebrazed part is formed such that, in a cross section of the brazed parttaken along a plane orthogonal to a direction in which the brazed partextends, a bead width of the brazed part on the sheet surface satisfiesFormula (2) below:t≤W≤4t  (2) where W: bead width (mm) of the brazed part t: sheetthickness (mm) of the bonding target sheet.
 3. The method as set forthin claim 1, wherein: in the brazing step, fluxless laser brazing iscarried out with use of a fluxless brazing filler metal as a brazingfiller metal.
 4. The method as set forth in claim 3, wherein: thebrazing filler metal is composed of Al; or the brazing filler metal hasa composition that contains Al and one or more selected from the groupconsisting of Si: 0.2% by mass to 15.0% by mass, Mg: 0.3% by mass to7.0% by mass, and Mn: 0.03% by mass to 2.0% by mass.
 5. The method asset forth in claim 1, wherein: the coating layer of the hot dip Zn-basedalloy coated steel sheet further contains Mg in an amount of 0.05% bymass to 10.0% by mass.
 6. The method as set forth in claim 1, wherein:the coating layer of the hot dip Zn-based alloy coated steel sheetfurther contains one or two or more selected from the group consistingof Ti: 0.002% by mass to 0.1% by mass, B: 0.001% by mass to 0.05% bymass, Si: 0% by mass to 2.0% by mass, and Fe: 0% by mass to 2.5% bymass.
 7. The method as set forth in claim 1, wherein: a coating weighton the sheet surface of the hot dip Zn-based alloy coated steel sheet is15 g/m² to 250 g/m².
 8. A method for producing a lap joint member bybrazing a bonding target sheet to a hot dip Zn-based alloy coated steelsheet with a method recited in claim 1.